In one way it seems to be possible. Like unloading the fglrx module is usally no big deal when X is down. But my attempts to switch from oss kms to fglrx where never successfull. I had the code in my script but disabled it (it works for nouveau -> nvidia). From oss without kms to fglrx the success rate was a bit better but not great. Just stop X, rmmod fglrx, switch those libs, run maybe ldconfig and move the xorg.conf away, then start X again.